Page retirement in a nand flash memory system

ABSTRACT

In a data storage system including a non-volatile random access memory (NVRAM) array, a page is a smallest granularity of the NVRAM array that can be accessed by read and write operations, and a memory block containing multiple pages is a smallest granularity of the NVRAM array that can be erased. Data are stored in the NVRAM array in page stripes distributed across multiple memory blocks. In response to detection of an error in a particular page of a particular block of the NVRAM array, only the particular page of the particular block is retired, such that at least two of the multiple memory blocks across which a particular one of the page stripes is distributed include differing numbers of active (non-retired) pages.

BACKGROUND OF THE INVENTION

This disclosure relates to data processing and storage, and morespecifically, to improving the ability of a data storage system toefficiently perform page retirement.

In certain data storage systems, data is stored in multiple storagedevices. For example, in some such systems, multiple individual harddisks or memory chips are used to store data, and the data stored in oneor more of the storage devices is associated with data stored in otherstorage devices in such a manner that data errors in one or more storagedevices can be detected and possibly corrected. One such approach is tostore a given quantity of data across multiple storage locations bydividing the quantity of data into portions of equal size—the individualportions sometimes being referred to as “data pages”—and then storingthe data pages in multiple storage locations such that one data page isstored in each of multiple storage devices. In connection with thisapproach, a further storage device may be used to store a page of dataprotection information, where a given page of data protectioninformation is associated with a specific set of data pages stored inthe multiple storage locations. In some instances, the set of data pagesin the multiple locations that is used to store associated data isreferred to as a “data stripe” or “page stripe.”

In addition to the data protection information for each data stripe,individual data pages may also be protected by an error correcting code(ECC) that may be utilized to detect errors and to correct some numberof errors within the page. ECC protection is provided on a certain codeword whose size is often referred to as the codeword (or block) length.There may be multiple codewords within a data page. At some number oroccurrence of errors, the data storage system may determine to withdrawfrom use (retire) portions of the data storage that are the source oferrors. In data storage systems employing NAND flash memory, a data pageis the smallest granule of storage that can be accessed by read andwrite operations, and a block (or erase block), which contains manypages, is the smallest granule of storage that can be erased.Consequently, it is conventional for a data storage system to retire anentire erase block from use in response to an ECC failure of even acodeword on a single page within the erase block.

The present disclosure recognizes that this conventional blockretirement policy is over-inclusive and can unnecessarily shorten thelife of a NAND flash storage device because the NAND flash storagedevice will itself be retired when a threshold number of its eraseblocks are retired.

BRIEF SUMMARY

In a data storage system including a non-volatile random access memory(NVRAM) array (e.g., a NAND flash memory array), a page is the smallestgranularity of the NVRAM array that can be accessed by read and writeoperations, and a memory block containing multiple pages is a smallestgranularity of the NVRAM array that can be erased. Data are stored inthe NVRAM array in page stripes distributed across multiple memoryblocks. In response to detection of an uncorrectable error in aparticular code word of a particular page of a particular block of theNVRAM array, only the particular page of the particular block isretired, such that at least two of the multiple memory blocks acrosswhich a particular one of the page stripes is distributed includediffering numbers of active (non-retired) pages.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a high level block diagram of a data processing environment inaccordance with one embodiment;

FIGS. 2-5 illustrate an exemplary arrangement of physical memory withina NAND flash memory array in accordance with the present disclosure;

FIG. 6 depicts an exemplary implementation of a page stripes inaccordance with the present disclosure;

FIG. 7 illustrates an exemplary data page in accordance with the presentdisclosure;

FIG. 8 depicts an exemplary data protection page in accordance with thepresent disclosure;

FIG. 9 is a high level logical flowchart of an exemplary embodiment of adata read process in which physical memory in a NAND flash memory arrayis retired on a page-by-page basis rather than on a block-by-blockbasis;

FIG. 10 is a high level logical flowchart of an exemplary embodiment ofa data write process by which data is written to a NAND flash memoryarray in a variable length page stripe across variable sized datablocks; and

FIG. 11 illustrates an exemplary page stripe in a NAND flash memoryarray in which physical memory is retired on a page-by-page basis.

DETAILED DESCRIPTION

As will be appreciated by one skilled in the art, aspects of the presentinvention may be embodied as a system, method or computer programproduct. Accordingly, aspects of the present invention may take the formof an entirely hardware embodiment, an entirely software embodiment(including firmware, resident software, micro-code, etc.) or anembodiment combining software and hardware aspects that may allgenerally be referred to herein as a “circuit,” “module” or “system.”Furthermore, aspects of the present invention may take the form of acomputer program product embodied in one or more computer readablemedium(s) having computer readable program code embodied thereon.

Any combination of one or more computer readable medium(s) may beutilized. The computer readable medium may be a computer readable signalmedium or a computer readable storage medium. A computer readablestorage medium may be, for example, but not limited to, an electronic,magnetic, optical, electromagnetic, infrared, or semiconductor system,apparatus, or device, or any suitable combination of the foregoing. Morespecific examples (a non-exhaustive list) of the computer readablestorage medium would include the following: an electrical connectionhaving one or more wires, a portable computer diskette, a hard disk, arandom access memory (RAM), a read-only memory (ROM), an erasableprogrammable read-only memory (EPROM or Flash memory), an optical fiber,a portable compact disc read-only memory (CD-ROM), an optical storagedevice, a magnetic storage device, or any suitable combination of theforegoing. In the context of this document, a computer readable storagemedium may be any tangible medium that can contain, or store a programfor use by or in connection with an instruction execution system,apparatus, or device.

A computer readable signal medium may include a propagated data signalwith computer readable program code embodied therein, for example, inbaseband or as part of a carrier wave. Such a propagated signal may takeany of a variety of forms, including, but not limited to,electro-magnetic, optical, or any suitable combination thereof. Acomputer readable signal medium may be any computer readable medium thatis not a computer readable storage medium and that can communicate,propagate, or transport a program for use by or in connection with aninstruction execution system, apparatus, or device.

Program code embodied on a computer readable medium may be transmittedusing any appropriate medium, including but not limited to wireless,wireline, optical fiber cable, RF, etc., or any suitable combination ofthe foregoing.

Computer program code for carrying out operations for aspects of thepresent invention may be written in any combination of one or moreprogramming languages, including an object oriented programming languagesuch as Java, Smalltalk, C++ or the like and conventional proceduralprogramming languages, such as the “C” programming language or similarprogramming languages. The program code may execute entirely on theuser's computer, partly on the user's computer, as a stand-alonesoftware package, partly on the user's computer and partly on a remotecomputer or entirely on the remote computer or server. In the latterscenario, the remote computer may be connected to the user's computerthrough any type of network, including a local area network (LAN) or awide area network (WAN), or the connection may be made to an externalcomputer (for example, through the Internet using an Internet ServiceProvider).

Aspects of the present invention are described below with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems) and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer program instructions. These computer program instructions maybe provided to a processor of a general purpose computer, specialpurpose computer, or other programmable data processing apparatus toproduce a machine, such that the instructions, which execute via theprocessor of the computer or other programmable data processingapparatus, create means for implementing the functions/acts specified inthe flowchart and/or block diagram block or blocks.

These computer program instructions may also be stored in a computerreadable medium that can direct a computer, other programmable dataprocessing apparatus, or other devices to function in a particularmanner, such that the instructions stored in the computer readablemedium produce an article of manufacture including instructions whichimplement the function/act specified in the flowchart and/or blockdiagram block or blocks.

The computer program instructions may also be loaded onto a computer,other programmable data processing apparatus, or other devices to causea series of operational steps to be performed on the computer, otherprogrammable apparatus or other devices to produce a computerimplemented process such that the instructions which execute on thecomputer or other programmable apparatus provide processes forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks.

With reference now to the figures and with particular reference to FIG.1, there is illustrated a high level block diagram of an exemplary dataprocessing environment 100 including a data storage system that retiresNAND flash memory on a page-by-page basis rather than a block-by-blockbasis, as described further herein. As shown, data processingenvironment 100 includes at least one processor system 102 having one ormore processors 104 that process instructions and data. Processor system102 may additionally include local storage 106 (e.g., dynamic randomaccess memory (DRAM) or disks) that may store program code, operandsand/or execution results of the processing performed by processor(s)104. In various embodiments, processor system 102 can be, for example, amobile computing device (such as a smartphone or tablet), a laptop ordesktop personal computer system, a server computer system (such as oneof the POWER series available from International Business MachinesCorporation), or a mainframe computer system. It can also be an embeddedprocessor system using various processors such as ARM, PowerPC, IntelX86, or any other processor combined with memory caches, memorycontrollers, local storage, I/O bus hubs, etc.

Processor system 102 further includes an input/output (I/O) adapter 108that is coupled directly (i.e., without any intervening device) orindirectly (i.e., through at least one intermediate device) to a datastorage system 120 via an I/O channel 110. In various embodiments, I/Ochannel may employ any one or a combination of known or future developedcommunication protocols, including, for example, Fibre Channel (FC), FCover Ethernet (FCoE), Internet Small Computer System Interface (iSCSI),Transport Control Protocol/Internet Protocol (TCP/IP), PeripheralComponent Interconnect Express (PCIe), etc. I/O operations (IOPs)communicated via I/O channel 110 include read IOPs by which processorsystem 102 requests data from data storage system 120 and write IOPs bywhich processor system 102 requests storage of data in data storagesystem 120.

In the illustrated embodiment, data storage system 120 includes aninterface 122 through which data storage system 120 receives andresponds to IOPs 102 via I/O channel 110. Interface 122 is coupled to aflash controller 124 (e.g., an Application Specific Integrated Circuit(ASIC) or Field Programmable Gate Array (FPGA)) having an associatedflash controller memory 126 (e.g., Dynamic Random Access Memory (DRAM)).Flash controller 124 is additionally coupled to a CPU 128 having anassociated CPU memory 130 (e.g., DRAM) and further coupled to a NANDflash memory array 140. In embodiments in which flash controller 124 isimplemented with an FPGA, CPU 128 may program and configure flashcontroller 124 during start-up of data storage system 120. Afterstartup, in general operation flash controller 124 receives read andwrite IOPs via I/O channel 110 and interface 122 to read data stored inNAND flash memory array 140 and/or to store data in NAND flash memoryarray 140. Flash controller 124 services these IOPs, for example, byaccessing NAND flash memory array 140 to read or write the requesteddata from or into NAND flash memory array 140 or by accessing a memorycache (not illustrated) associated with NAND flash memory array 140.

Flash controller 124 implements a flash translation layer (FTL) thatprovides logical-to-physical address translation to enable access tospecific memory locations within NAND flash memory array 140. Ingeneral, an IOP received by flash controller 124 from a host device,such as a processor system 102, contains the logical block address (LBA)at which the data is to be accessed (read or written) and, if a writeIOP, the write data to be stored to data storage system 120. The IOP mayalso specify the amount (or size) of the data to be accessed. Otherinformation may also be communicated depending on the protocol andfeatures supported by data storage system 120. As noted above, NANDflash memory, such as that employed in NAND flash memory array 140, isconstrained by its construction such that the smallest granule of datathat can be accessed by a read or write IOP is fixed at the size of asingle flash memory page, for example, 16 kilobytes (kB). The LBAprovided by the host device thus corresponds to a page within a logicaladdress space. The flash translation layer translates this LBA into aphysical address assigned to a corresponding physical location in NANDflash memory array 140. Flash controller 124 may perform addresstranslation and/or store mappings between logical and physical addressesin a logical-to-physical translation data structure, such as translationtable 150, which may conveniently be stored in flash controller memory126.

In some embodiments, the components of data storage system 120 aremounted to the same printed circuit board, for example, using surfacemounting techniques, through-hole techniques, through the use of socketsand socket-mounts and/or other mounting techniques. In such embodiments,data storage system 120 may optionally include one or more on-boardcapacitors 132 that automatically supplies power to data storage system120 in the event of a power failure. Data storage system 120 mayconsequently serve as a non-volatile memory system, even though itutilizes various volatile components.

NAND flash memory array 140 may take many forms in various embodiments.Referring now to FIGS. 2-5, there is depicted one exemplary arrangementof physical memory within a NAND flash memory array 140 in accordancewith the present disclosure.

As shown in FIG. 2, NAND flash memory array 140 may be formed fromthirty-two (32) individually addressable NAND flash memory storagedevices. In the illustrated example, each of the flash memory storagedevices M0 a-M15 b takes the form of a board-mounted flash memorymodule, for example, a Single Level Cell (SLC) or Multi-Level Cell (MLC)NAND flash memory module. The thirty-two NAND flash memory modules arearranged in sixteen groups of two (M0 a, M0 b) through (M15 a, M15 b).For purposes of the physical addressing scheme, each group of twomodules forms a “lane,” also sometimes referred to as a “channel,” suchthat NAND flash memory array 140 includes sixteen channels or lanes(Lane0-Lane15).

In a preferred embodiment, each of the individual lanes has a respectiveassociated bus coupling it to flash controller 124. Thus, by directingits communications to one of the specific communication buses, flashcontroller 124 can direct its communications to one of the lanes ofmemory modules. Because each communication bus for a given lane isindependent of the communication buses for the other lanes, flashcontroller 124 can issue commands and send or receive data across thevarious communication buses at the same time, enabling flash controller124 to access the memory modules corresponding to the individual lanesat, or very nearly at, the same time.

With reference now to FIG. 3, there is illustrated an exemplaryembodiment of a flash memory module 300 that can be utilized toimplement any of flash memory modules M0 a-M15 b of FIG. 2. As shown inFIG. 3, the physical storage locations provided by flash memory module300 are further subdivided into physical locations that can be addressedand/or identified through Chip Enables (CEs). In the example of FIG. 3,the physical memory of each flash memory chip 300 is divided into fourChip Enables (CE0, CE1, CE2 and CE3), each having a respective CE linethat is asserted by flash memory controller 124 to enable access to orfrom the physical memory locations within the corresponding CE. Each CEis in turn subdivided into multiple dice (Die0 and Die1) each having twoplanes (Plane0 and Plane1). Each plane represents a collection of blocks(described below) that, because of the physical layout of the flashmemory chips, are physically associated with one another and thatutilize common circuitry (e.g., I/O buffers) for the performance ofvarious operations, such as read and write operations.

As further shown in FIGS. 4-5, an exemplary plane 400, which can beutilized to implement any of the planes within flash memory module 300of FIG. 3, includes 2048 blocks of physical memory. In general, a block500 is a collection of physical pages that are associated with oneanother, typically in a physical manner. This association is such that ablock is defined to be the smallest granularity of physical storagelocations that can be erased within NAND flash memory array 140. In theembodiment of FIG. 5, each block 500 includes 256 physical pages, wherea physical page is defined to be the smallest individually addressabledata unit for read and write access. In the exemplary system, eachphysical page of data has a common capacity (e.g., 16 kB) for datastorage plus additional storage for metadata described in more detailbelow. Thus, data is written into or read from NAND flash memory array140 on a page-by-page basis, but erased on a block-by-block basis. Asfurther shown in FIG. 5, each block 500 preferably includes page statusinformation 502, which indicates the status of each physical page inthat block 500 as retired (i.e., withdrawn from use) or non-retired(i.e., active or still in use). In various implementations, PSI 502 canbe collected into a single data structure (e.g., a vector or table)within block 500, distributed within block 500 (e.g., as one or morebits of metadata appended to each physical page) or maintained elsewherein data storage system 120 (e.g., in a data structure in flashcontroller memory 126).

As noted above, data storage system 120 does not generally allowexternal devices to directly address and/or access the physical memorylocations within NAND flash memory array 140. Instead, data storagesystem 120 is generally configured to present a single contiguouslogical address space to the external devices, thus allowing hostdevices to read and write data to and from LBAs within the logicaladdress space while permitting flash controller 124 and CPU 128 tocontrol where the data that is associated with the various LBAs actuallyresides in the physical memory locations comprising NAND flash memoryarray 140. In this manner, performance and longevity of NAND flashmemory array 140 can be intelligently managed and optimized.

As writes occur to data storage system 120, it will be necessary for theflash controller 124 and CPU 128 to invalidate the data stored in one ofthe physical pages of one of the erase blocks in the NAND flash memoryarray 140. The new data will then be coalesced with data being writtenand eventually stored in different location in NAND flash memory array140. It can be seen then that pages or portions of pages will beinvalidated and therefore portions of the NAND Flash memory array 140become unused. Flash controller 124 and CPU 128 will eventually need toreclaim this space through a process called Garbage Collection.Particular erase blocks will be chosen based on a number of factorsincluding how much of the data within the erase blocks is invalid. Validdata will be read and written along with new writes from the host intonew erase blocks.

Because the flash translation layer implemented by data storage system120 isolates the logical address space made available to host devicesfrom the physical memory within NAND flash memory array 140, the size ofNAND flash memory array 140 need not be equal to the size of the logicaladdress space presented to host devices. In most embodiments it isbeneficial to present a logical address space that is less than thetotal available physical memory (i.e., to over-provision NAND flashmemory array 140). Over-provisioning in this manner ensures thatphysical memory resources are available when the logical address spaceis fully utilized, even given the presence of a certain amount ofinvalid data as described above. In addition to invalid data that hasnot yet been reclaimed the overprovisioned space can be used to ensurethere is enough logical space even given the presence of memory failuresand the memory overhead entailed by the use of data protection schemes,such as Error Correcting Code (ECC), Cycle Redundancy Check (CRC), andparity.

In a preferred embodiment, data is typically written to groups ofassociated physical pages of NAND flash memory array 140 referred toherein as “page stripes.” While the lengths of the various page stripesstored into NAND flash memory array 140 can and preferably do vary, inone embodiment each page stripe includes two to fifteen data pages ofwrite data (typically provided by a host device) and one additional page(a “data protection page”) used to store data protection information forthe write data. For example, FIG. 6 illustrates an exemplary page stripe600 including N data pages (i.e., Dpage00 through DpageN−1) and one dataprotection page (i.e., PpageN).

FIG. 7 illustrates an exemplary format of each data page within the pagestripe 700. In this example, data page 700 includes a 16 kB data field702, as well as additional fields for metadata describing the data page.In the illustrated example, these metadata fields include an LBA field704 containing the LBA of data page 700, a CRC field 706 containing theCRC value computed for the combination of data field 702 and LBA field704, and an ECC field 708 containing an ECC value calculated, in theillustrated example, from a combination of contents of data field 702,LBA field 704 and CRC field 706.

FIG. 8 depicts an exemplary format of the data protection page of pagestripe 800. In the depicted example, data protection page 800 includes adata XOR field 802 that contains the bit-by-bit Exclusive Or (XOR) ofthe contents of the data fields 702 of the data pages 700 in page stripe600. Data protection page 800 further includes an LBA XOR field 804 thatcontains the bit-by-bit XOR of the LBA fields 704 of the data pages 700in page stripe 600. Data protection page 800 finally includes a CRCfield 806 and ECC field 808 for respectively storing a CRC value and anECC value for data protection page 800. Such a protection scheme iscommonly referred to as RAID 5, since the parity field will not alwaysbe located on one particular flash plane. However, it should beappreciated that alternate data protection schemes such as Reed-Solomoncan be used.

The formats for data pages and data protection pages described aboveprotect data stored in a page stripe using multiple different protectionmechanisms. First, the use of the ECC bits in each data page allows thecorrection of some number of bit errors within the flash page. Dependingon the ECC method used it may be possible correct hundreds of bits oreven thousands of bits within a NAND flash page. After ECC checking andcorrection is performed, the corrected CRC field is used to validate thecorrected data. Used together, these two mechanisms allow for thecorrection of relatively benign errors and the detection of more seriouserrors using only local intra-page information. Should an uncorrectableerror occur in a data page, for example, due to failure of the physicalpage utilized to store the data page, the contents of the data field andLBA field of the failing data page may be reconstructed from the otherdata pages and the data protection page for the page stripe.

While the physical memory locations in which the data pages and dataprotection page of a page stripe will vary within NAND flash memoryarray 140, in one embodiment the data pages and data protection pagethat comprise a given page stripe are preferably stored in physicalmemory locations selected to optimize the overall operation of the datastorage system 120. For example, in some embodiments, the data pages anddata protection page comprising a page stripe are stored such thatdifferent physical lanes are employed to store each of the data page anddata protection page. Such embodiments support efficient access to apage stripe because flash controller 124 can access all of the pages ofdata that comprise the page stripe simultaneously or nearlysimultaneously. It should be noted that the assignment of pages to lanesneed not be sequential (i.e., data pages can be stored in any lane inany order), and unless a page stripe is a full length page stripe (e.g.,containing fifteen data pages and one data protection page), the lanesutilized to store the page stripe need not be adjacent.

Having described the general physical structure of the data storagesystem 120 and aspects of the manner in which data in the form of pagestripes are addressed and stored within NAND flash memory array 140,certain operational aspects of data storage system 120 are nowdescribed, including aspects relating to reading and writing data fromand to NAND flash memory array 140.

With reference now to FIG. 9, there is illustrated a high level logicalflowchart of an exemplary method of performing a read operation in datastorage system 120 in which physical memory may be retired on a pagebasis. The method may be performed, for example, by flash controller 124and/or CPU 128 in hardware, firmware, software or a combination thereof.For simplicity of explanation, it will hereafter be assumed that theprocess is performed by flash controller 124. As with the otherflowcharts provided herein, steps are presented in logical rather thanstrictly chronological order, and in some embodiments at least some ofthe steps may be performed in an alternative order or concurrently.

The illustrated process begins at block 900 and then proceeds to block902, which illustrates flash controller 124 awaiting receipt of a readIOP from an external host device, such as processor system 102. Ingeneral, the read IOP will specify the LBA of a target data page that isrequested by the host device. In response to receipt of the read IOP,flash controller 124 translates the LBA (e.g., by reference tological-to-physical translation table (TT) 150 in flash controllermemory 126) to determine the physical address of the physical page thatstores the requested data page (block 904). Once the physical address isdetermined, flash controller 124 accesses the target data page utilizingthe physical address by issuing a read request to read the target datapage from the physical page associated with the physical address (block906). The read IOP may request various sizes of data, but forsimplification it is assumed that the flow diagram 900 will be exercisedonce for each logical page referenced by the read IOP.

At block 910, flash controller 124 computes the ECC for the data pageand compares the computed ECC with the ECC contents of ECC field 708 todetermine whether or not the data page contains a data error. Such dataerrors can be due to any number of problems, including trapped charge inthe oxide layer of the physical device or charge that has leaked fromthe floating gate. These errors may be permanent in nature such that thecell is no longer capable of storing and reading data correctly or theerrors may be due to issues related to data retention or electricalnoise inflicted by writes or reads of adjacent cells. Such errors willnot be present when the cell is erased and then rewritten. True softerrors are substantially random and are typically not uniquelyassociated with any specific physical pages, blocks or other physicalregions of NAND flash memory array 140. A hard error, on the other hand,is a corruption of one or multiple bits of data caused by a physicalfailure. Hard errors can be caused by a variety of factors including,but not limited to, the physical failure of one or more componentswithin a given memory chip (such as the failure of a charge pump), thephysical failure of an entire memory chip or the external supportstructures for that chip (e.g., the breaking of a power line or anaddress line to a chip), the physical failure of all or part of a chipas a result of environmental factors (e.g., excessive temperature,magnetic field, humidity, etc). In general, because hard errors arisefrom the physical structure of the memory system, hard errors areuniquely associated with a particular collection of memory chips, aparticular memory chip, or specific physical regions within a chip (suchas a CE, plane, block or page).

In response to a determination at block 910 that the data page does notcontain a data error, the process passes to block 930, which isdescribed below. However, in response to a determination at block 910that the data page contains a data error, flash controller 124 correctsthe error in the data page utilizing the ECC, if possible (e.g., ifthere are 50 bits in error within the codeword and the ECC is capable ofcorrecting greater than 50 bits in error within the code word.). Atblock 914, flash controller 124 determines whether or not the detecteddata error is a hard error. Flash controller 124 can infer that a dataerror is a hard error, for example, based on a frequency or number ofdata errors from a given physical memory region. In other words, flashcontroller 124 may notice that all physical pages within a given eraseblock have failed. In response to a determination at block 914 that thedata error is a correctable error, the process proceeds to block 930,which is described below. If, on the other hand, flash controller 124determines that the data error is an uncorrectable error, the processpasses to block 916.

At block 916, flash controller 124 marks only the physical page thatstores the target data page as retired (i.e., no longer available forallocation to store a data page of a new page stripe), for example, inthe PSI 502 of the block containing that physical page. Thus, incontrast to prior art systems that retire physical memory in a NANDflash memory on a block-by-block basis, data storage system 120 retiresphysical memory in NAND flash memory array 140 on a page-by-page basisin response to detection of a single data error. As will be appreciated,retirement of a physical page of physical memory (e.g., 16 kB) ratherthan a block (e.g., 4 MB) conserves physical memory resources, enhancingthe performance and extending the life of NAND flash memory array 140.However, as a consequence of page retirement, the effective sizes ofblocks of physical memory of NAND flash memory system 140 will vary, asdescribed further below.

This exemplary implementation waits until a physical page has one ormore codewords containing uncorrectable errors before performing theretirement. However, certain implementations may choose to retire a pageat some point prior to uncorrectability. For example, if animplementation uses BCH ECC over approximately 1024 bytes and cancorrect 50 bits in error, flash controller 124 may decide to retire apage when the number of uncorrectable bits reaches a number less than50, say 48. Additionally, one skilled in the art will also know that,depending on the flash technology used, that flash controller 124 mayelect to perform additional steps at block 914 before retiring the page.For example, the flash manufacturer may require flash controller 124 tochange certain parameters for that page or block and perform a re readof the page. If the data is now correctable, then flash controller 124would follow the No path from block 914. In this way, block 914 maycontain many additional steps not described herein in determining thatthe page contains a hard error or truly uncorrectable error.

At block 918, flash controller 124 further determines by reference toPSI 502 whether or not the number of retired pages of physical memory ina physical memory region containing the page retired at block 916 nowsatisfies (e.g., is greater than and/or equal to) a first threshold T1.In at least some embodiments, the first threshold T1 is configurable.Performance testing has shown that performance of data storage system120 is optimized if the first threshold T1 is less than about 10% of thephysical pages in the physical memory region and, more particularly, isconfigured to be about 5% of the physical pages in the physical memoryregion. Still more particularly, it is preferable if the first thresholdT1 is configured to be about 4% of the physical pages in the physicalmemory region. These numbers are exemplary only and can varydramatically based on the use and requirements of the data storagesystem. The physical memory region to which the first threshold T1pertains can be, for example, a block, plane, die, CE or entire flashmodule.

In response to flash controller 124 determining at block 918 that thefirst threshold is not satisfied, the process passes to block 930.However, in response to flash controller 124 determining at block 918that the first threshold is satisfied, flash controller 124 marks theentire physical memory region containing the retired physical page asretired and thus unavailable for allocation to store new page stripes(block 920). If the physical memory region for which retirement isdetermined at block 918-920 is a smaller region than an entire flashmemory module, flash controller 124 may optionally determine whether ornot to retire larger physical memory regions containing the retiredphysical page, as shown at optional blocks 922-924. As indicated, theadditional determination regarding retirement of larger physical memoryregions can be based on a comparison of the number of retired physicalmemory regions (e.g., pages, blocks, planes, dies or CEs) within thelarger physical memory region and a second threshold T2 (which maydiffer from threshold T1). The process proceeds from either block 920 orone of blocks 922 and 924 to block 930. It may also be determined thatthere are now too many retired memory resources within NAND flash memorysystem 140 to be able to achieve required performance levels, and flashcontroller 124 may send a response to the user indicating that it istime to replace the physical resource containing NAND flash memorysystem 140.

At block 930, flash controller 124 attempts to validate the data field702 and LBA field 704 of the target data page by computing a cyclicredundancy code and comparing the computed CRC to that contained in CRCfield 706. In response to successful validation of the data page, theprocess proceeds to block 934, which is described below. However, inresponse to failure of the CRC validation, flash controller 124 readsthe entire page stripe and recovers the correct content of the targetdata page from the other data page(s) and the data protection page ofthe page stripe (block 932). In addition, flash controller 124 moves thepage stripe (including the recovered target data page) to a differentphysical location in NAND flash memory system 120, for example,utilizing the method depicted in FIG. 10, which is described below. Fromblock 930 or block 932, the process proceeds to block 934, whichillustrates flash controller 124 transmitting the target data page tothe requesting host device. Thereafter, the process of FIG. 9 terminatesat block 940. The method of FIG. 9 may again be performed at some laterunspecified time depending on other operations in process.

As noted above, the order of operations set forth in FIG. 9 is exemplaryand embodiments are envisioned where the order of operations isdifferent from that set out above. For example, embodiments areenvisioned in which flash controller 124 transmits the data page to thehost device prior to completing CRC validation and thereafter transmitsan indication of whether the data page is valid or not. Also in someembodiments, the read TOP may access multiple data pages rather than asingle data page.

With reference now to FIG. 10, there is illustrated a high level logicalflowchart of an exemplary method of performing a write operation in datastorage system 120. The method may be performed, for example, by flashcontroller 124 and/or CPU 128 in hardware, firmware, software or acombination thereof. For simplicity of explanation, it will hereafter beassumed that the process is performed by flash controller 124.

The method begins at block 1000 and then proceeds to block 1002, whichillustrates flash controller 124 awaiting receipt of a write TOP from ahost device, such as processor system 102. The write TOP includes, forexample, a data to be written into NAND flash memory array 140 and anindication of the target LBA(s) at which the host device would like forthe data to be stored. In response to receipt of a write TOP, theprocess proceeds from block 1002 to blocks 1004-1106, which illustratesflash controller 124 determining (e.g., by reference tological-to-physical address translation table 150) whether the targetLBA(s) indicated in the write TOP is/are currently mapped to physicalmemory page(s) and, if so, changing the status of each data pagecurrently associated with a target LBA to indicate that it is no longervalid. As is known in the art, invalidated pages (and the blockscontaining them) will eventually be erased and again made available forallocation to store data by a garbage collection process of flashcontroller 124.

At block 1008, flash controller 124 allocates physical pages to form apage stripe to store the write data. As noted above, flash controller124 preferably selects each of the physical pages allocated to storewrite data from a different block in a different lane. Further, inallocating the physical pages to store the page stripe, flash controller124 excludes from consideration any retired pages, as indicated by thePSIs 502. As indicated at block 1010, flash controller 124 then beginsto store the first data page of the write data to the first physicalpage allocated to the page stripe. At block 1012, flash controller 124additionally computes the CRC and ECC values for the data page andstores that those value into the CRC field 706 and ECC field 708 of thedata page. As indicated by block 1014, flash controller 124 determineswhether all of the write data has been written into NAND flash memorysystem 140. If flash controller 124 determines that less than all of thewrite data has been written to NAND flash memory system 140, the processreturns to block 1010, which has been described. In response to adetermination that all of the write data has been stored in NAND flashmemory system 140, flash controller 124 computes parity information forthe page stripe (e.g., by performing a bit-by-bit XOR of all the datapages in the page stripe) and stores the parity information in the dataprotection page of the page stripe (block 1016). Again, any physicalpages that are retired that would otherwise be allocated in the datastripe are excluded from the parity computation. Flash controller 124also updates the logical-to-physical address translation table toassociate the physical page(s) utilized to store the write data with theLBA(s) indicated by the host device (block 1018). Thereafter, theprocess ends at block 1020.

It should be noted that blocks 1008, 1010 and 1012 will include stepsthat check whether or not the programming of that physical pagecompletes correctly. If flash controller 124 determines that the writedid not complete correctly, then flash controller 124 will employvarious recovery techniques, such as retrying the write operation. Ifthe write eventually passes, then flash controller 124 will continue tothe next block. However, if the write is unsuccessful, then flashcontroller 124 will retire that page in the same manner as when a readis unsuccessful. Having retired that page, flash controller 124 willchange its tables accordingly and proceed to the next non-retired pageto complete either blocks 1008,1010 or 1012.

With reference now to FIG. 11, there is illustrated an exemplary pagestripe 1102 written to NAND flash memory system 140 in accordance withthe method of FIG. 10. As noted above, page stripes written to NANDflash memory system 140 can vary in length between two and fifteen datapages plus one data protection page. However, in the illustratedexample, flash controller 124 has written a page stripe including sixdata pages and one data protection page. In this example, flashcontroller 124 has selected the physical pages for storing the pagestripe from a pool of blocks including blocks 1100 a-1100 h in lanes0-7, respectively. In each of blocks 1100 a-1100 h, one or more pageshave been retired, as graphically indicated in FIG. 11 by an “X” and asrecorded by flash controller 124 in PSI 502. Thus, for example, Page1,Page123 and Page255 of block 1100 a are retired; Page1 and Page86 ofblock 1100 g are retired; and Page23 of block 1100 h is retired.

In flash memory, it is beneficial to write data into blocks sequentiallyby page in some order determined by the flash manufacturer. Thus, whenwriting to a block of NAND flash memory system 140, it may be desirableand beneficial to write first to Page1, then to Page1 and so on untilthe block is full, or nearly full, of stored data. In someimplementations it may be desirable to write in some order other thanPage1, Page1, etc., but the end result is still a sequential processingof pages in some order for the entire block. In the embodiment of FIG.11, this form of writing data into NAND flash memory 140 is accomplishedby having flash controller 124 maintain a count so that the first pagestripe written into a given group of blocks is written across Page1 ofthe blocks within the group of blocks, the next page stripe is writtenacross Page1 of the blocks within the group of blocks, and so on. Itshould be noted, however, that any retired pages, such as Page1 of block1100 g, are excluded from allocation to a page stripe, such as pagestripe 1102. Similarly, when the next page stripe is written to thegroup of blocks, Page1 of block 1100 a will not be used.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof code, which comprises one or more executable instructions forimplementing the specified logical function(s). It should also be notedthat, in some alternative implementations, the functions noted in theblock may occur out of the order noted in the figures. For example, twoblocks shown in succession may, in fact, be executed substantiallyconcurrently, or the blocks may sometimes be executed in the reverseorder, depending upon the functionality involved. It will also be notedthat each block of the block diagrams and/or flowchart illustration, andcombinations of blocks in the block diagrams and/or flowchartillustration, can be implemented by special purpose hardware-basedsystems that perform the specified functions or acts, or combinations ofspecial purpose hardware and computer instructions.

As has been described, in some embodiments of a data storage systemincluding a NAND flash memory array, a page is a smallest granularity ofthe NAND flash memory array that can be accessed by read and writeoperations, and a memory block containing multiple pages is a smallestgranularity of the NAND flash memory array that can be erased. Data arestored in the NAND flash memory array in page stripes distributed acrossmultiple memory blocks. In response to detection of an error in aparticular page of a particular block of the NAND flash memory array,only the particular page of the particular block is retired, such thatat least two of the multiple memory blocks across which a particular oneof the page stripes is distributed include differing numbers of active(non-retired) pages.

While the present invention has been particularly shown as describedwith reference to one or more preferred embodiments, it will beunderstood by those skilled in the art that various changes in form anddetail may be made therein without departing from the spirit and scopeof the invention. For example, although aspects have been described withrespect to a data storage system including a flash controller thatdirects certain functions, it should be understood that presentinvention may alternatively be implemented as a program productincluding a storage device storing program code that can be processed bya processor to perform such functions or cause such functions to beperformed. As employed herein, a “storage device” is specificallydefined to include only statutory articles of manufacture and to excludetransitory propagating media per se.

In addition, although embodiments have been described that include useof a NAND flash memory, it should be appreciated that the inventionsherein are not limited to use with NAND flash memory, but are insteadapplicable to any other non-volatile random access memory (NVRAM)technology that may define erase blocks that are larger than physicalpages. For example, the disclosed techniques may be applied tophase-change memory (PCM), magnetoresistive RAM (MRAM) and resistive RAM(RRAM).

The figures described above and the written description of specificstructures and functions below are not presented to limit the scope ofwhat Applicants have invented or the scope of the appended claims.Rather, the figures and written description are provided to teach anyperson skilled in the art to make and use the inventions for whichpatent protection is sought. Those skilled in the art will appreciatethat not all features of a commercial embodiment of the inventions aredescribed or shown for the sake of clarity and understanding. Persons ofskill in this art will also appreciate that the development of an actualcommercial embodiment incorporating aspects of the present inventionswill require numerous implementation-specific decisions to achieve thedeveloper's ultimate goal for the commercial embodiment. Suchimplementation-specific decisions may include, and likely are notlimited to, compliance with system-related, business-related,government-related and other constraints, which may vary by specificimplementation, location and from time to time. While a developer'sefforts might be complex and time-consuming in an absolute sense, suchefforts would be, nevertheless, a routine undertaking for those of skillin this art having benefit of this disclosure. It must be understoodthat the inventions disclosed and taught herein are susceptible tonumerous and various modifications and alternative forms. Lastly, theuse of a singular term, such as, but not limited to, “a,” is notintended as limiting of the number of items.

What is claimed is:
 1. A method of page retirement in a data storagesystem including a non-volatile random access memory (NVRAM) array, themethod comprising: storing data in the NVRAM array in page stripesdistributed across multiple memory blocks, wherein at least two of themultiple memory blocks across which one of the page stripes isdistributed include differing numbers of active physical pages, whereina physical page is a smallest granularity that can be accessed in theNVRAM array and a memory block containing multiple physical pages is asmallest granularity that can be erased in the NVRAM array; in responseto detection of an error, retiring only a particular physical page of aparticular block in which the error occurred and recording retirement ofthe particular physical page in a page status data structure; andthereafter, allocating a plurality of page stripes across a group ofmemory blocks including the particular block such that each of theplurality of page stripes is formed at a respective one of a pluralityof different physical page indices, wherein the allocating includesskipping the particular page in the particular block when allocating afirst page stripe at a first physical page index based on the pagestatus data structure indicating the particular page as retired, andskipping another page in another block within the group of memory blockswhen allocating a second page stripe at a second physical page indexbased on the page status data structure indicating said another page asretired.
 2. The method of claim 1, wherein at least two of the multiplepage stripes are distributed across differing numbers of blocks.
 3. Themethod of claim 1, wherein the detecting includes detecting a mismatchbetween a code computed for the particular physical page and a codestored in the NVRAM array in association with the particular physicalpage.
 4. The method of claim 1, and further comprising: thereafter,retiring a physical memory region in the NVRAM array containing theparticular physical page and multiple other physical pages in responseto retirement of a threshold number of physical pages within thephysical memory region.
 5. The method of claim 4, wherein the physicalmemory region comprises the particular block.